1. Field of the Invention
The present invention generally relates to physiological monitoring devices. More particularly, this invention relates to a screening device and method for monitoring the occurrence of sleep disorder breathing events (e.g., decreases of upper respiratory airflow lasting more than ten seconds) in a fully ambulatory subject.
2. Description of the Related Art
The diagnosis of a patient's sleep disorders often involves the analysis of the patient's sleep-related events, such as sleep disordered breathing. Such breathing disorders may involve pauses in breathing. These are defined by the American Sleep Disorder Association and the American Sleep Apnea Association as being sleep "apnea" if the pause lasts ten or more seconds, with these being further identified as: (1) Central Apnea--cessation of airflow (upper airway--oral and nasal) and respiratory effort (amplitude of chest movement during breathing); (2) Obstructive Apnea--cessation of airflow with continuation of respiratory effort; and (3) Hypopnea--decrease in airflow from baseline (typically one-third to one-half or more) with continuation of normal or decreased levels of respiratory effort. These events lead to decreased blood oxygenation and thus to disruption of sleep.
Such events are often analyzed by the use of polysomnography (PSG), the monitoring and recording over an extended period of time of the temporal variations in the amplitude of the patient's sleep-impacted, physiological parameters, including: heart rate, eye blink activity, upper respiratory airflow, thorax and abdomen respiration efforts, the blood's oxygen saturation level, electroencephalograms (EEG; electrical activity of the brain), electro-oculogram (EOG; electrical activity related to movement of the eyes), and electromyograms (EMG; electrical activity of a muscle).
FIG. 1 displays portions of PSG computer tracings for three distinct, thirty-second intervals. Each of these printouts, showing the outputs from four of the system's sensors (i.e., upper respiratory airflow, thorax and abdomen respiration effort, and thorax activity)., illustrates temporal variations which are representative of the sleep-related apnea events defined above: FIG. 1A--Central Apnea, where the airflow sensor shows a marked cessation of airflow which lasted ten or more seconds, with cessation of respiratory effort indicated by the thorax and abdomen sensors; FIG. 1B--Obstructive Apnea, where the airflow signal ceases for ten or more seconds, but the thorax and abdomen sensors show no reduction in respiratory efforts; and FIG. 1C--Hypopnea, where airflow decreases by onethird or more for ten or more seconds and respiratory efforts are also significantly decreased.
PSG monitoring activities are expensive as they are typically conducted in clinical settings by trained PSG technicians who utilize expensive monitoring equipment having multiple sensors that are tethered to a centralized recording system and power supply.
More recently, a number of portable recording systems for diagnosing sleep disorders have been marketed. These systems range from multi-channel, PSG-style systems to much simpler units that monitor only one or more of the possible physiological parameters of interest. However, these multi-channel, portable systems remain technically complex, expensive and usually require trained PSG technicians to supervise their use.
All of the current, portable sleep testing systems share common, less-than-desirable features: (1) their use is expensive, since the equipment itself is expensive and a technician usually must be involved for its set-up and disconnection, plus the data collected must be subjectively analyzed by highly trained, sleep professionals, and (2) the PSG systems are bedside portable, but their use requires patients to be outfitted with an array of tethered electrode wires and sensors for connection to bulky body monitors or table-top consoles; thus, their size and weight does not allow the patient to be ambulatory, which can be essential for diagnosing patients' problems such as excessive sleepiness, or in the evaluation of treatment efficiency and compliance.
Less expensive alternatives, with capabilities for unattended, ambulatory use, are needed for assisting with the diagnosis of patients suffering from sleep disorders. For example, an inexpensive preliminary screening device, that provides information as to which patients might benefit the most from complete PSG testing, could contribute greatly by effectively expanding the audience to whom PSG testing would be available.
Recognizing the need for an improved apparatus or method for diagnosing of the sleep disorders of a fully ambulatory subject, it is therefore a general object of the present invention to provide a novel method and ambulatory, non-tethered apnea screening device to meet such needs.